1. Field of the Invention
This invention relates to a control method for a three-phase self-excited inverter, which is fed from a dc voltage source, commutated on the dc side, and in which the fundamental oscillation of the output voltage is adjustable in frequency and amplitude by means of pulsewidth modulation. The invention finds application in rectifier-fed drives, particularly in vehicles on rails. Also, it is understood that the "dc voltage source" feeding the inverter can be an intermediate dc voltage circuit of a converter such as, e.g., one converting a single-phase ac voltage into three-phase ac.
2. Description of the Prior Art
In the known methods for control of inverters by the subharmonic method, two different types can be distinguished, as now described.
In a first known method, the switching points of the inverter phases are derived from the intersections of a sinusoidal, rectangular or trapezoidal reference voltage and a triangular voltage of higher frequency (See BBC reprint "BBC Converter Technique for Drives", April 1970, Article 3, pp. 23 to 25 (Order No. GIA 60275), and also Article 2 of this publication, as well as BBC reprint from the Brown, Boveri Mitteilungen of January 1973, vol. 60, "The Development of Different Control Techniques of the Subharmonic Method Over Inverter Output Voltage" (Publication Order No. D ZEK 30388 D), 11 pp). The reference voltage there described may or may not be synchronized with the triangular voltage. The phase voltages may not only be changed between positive and negative polarities of a voltage but may assume, besides a zero value, two different voltage amplitudes of both polarities (first cited BBC reprint, Art. 3, FIGS. 3 and 4). Thereby the harmonics can be considerably reduced by simple control measures, which are called for when the component inverters used for the separate phases in the known case are reproducibly switched. The corresponding switching times of the inverter are determined by the superposition of the aforementioned voltages. Fundamental frequency and amplitude of the three-phase output voltage of the inverter are thereby given by the reference voltages, while the timing frequency is determined by the triangular voltage. One thus proceeds from a fixed frequency of the triangular voltage and the, e.g. sinusoidal, reference is varied. The result, therefore, is that the position of the switching time within the positive or negative half-cycle of the inverter voltage phase is not limited to a specific region of the half-cycle. Accordingly, this approach is characterized by what is called "free timing".
In another known method the switching times of the inverter voltage phase are so located that one or more harmonics do not appear (Journal "ELEKTRIE" 29 (1975) No. 1, pp. 35 to 37; IEEE Transactions on Industrial Applications, Vol. Ia-10, No. 1, January/February 1974, pp. 117 to 122; ETZ-A, Vol. 93 (1972), No. 9, pp. 528 to 530; IEEE Transactions on Industrial Applications, Vol. Ia-9, No. 3, May/June 1973, pp. 310 to 317 as well as No. 5, September/October 1974, pp. 666 to 673). In this known control method the position of the switching angle (i.e. the pulses of the inverter voltage) depends on the harmonics eliminated.
The same holds for still another known method in which there are always impressed on a 120.degree. block of a given phase missing current blocks from the other otherwise currentless phases of the machine, which occurs cyclically for all 120.degree. blocks of all phases. This current block of 120 electrical degrees duration is divided into separate blocks symmetric about the center line thereof. The minimum width of the undivided mean current block is 60 electrical degrees (ETZ-A, Vol. 9, (1976), No. 11, pp. 663 to 667 and DT-OS No. 2 531 255).
In the first mentioned type of method with "free timing", relatively many switching per phase are to be preferred. In the second type, a relatively costly control arrangement results in case variation of the harmonics to be eliminated is desired.
In yet another known method, as disclosed in U.S. Pat. No. 3,916,285 to Iwata et al, a set of timed gate pulse series for control of a pulse width modulated thyrister inverter is generated through the steps of generating three phased series of triangular waveform signals, the repetition frequencies of which are proportional to the level of an input command and which have a ratio therebetween of 4:2:1. The triangular waveform signals are level-compared with an input command, and on the basis of the crossover points therebetween, three series of pulses are generated having a pulse width ratio therebetween of 1:2:4. Then, depending upon the level of the input command, one of the three series of generated pulses is selected and converted to a set of desired time gate pulse series for control of the thyristor inverter. In the case where a three-phase pulse width modulated thyristor inverter is controlled, three desired sets of gate pulse series having a phase shift of 120.degree. therebetween is derived as the final step of the conversion of the series of pulses produced by the crossover comparisons.